The present invention is concerned with stud welding apparatus for the automatic processing of welding studs, comprising a housing, a spring-loaded stud holder which holds the welding stud in welding position by its resilient restoring force, a feed channel through which the stud is delivered into a substantially cylindrical space within the stud holder, and loading means which is movable relatively to the welding stud by a pneumatic piston and which presses said stud into the stud holder against the resilient restoring force.
Stud welding apparatuses of the kind referred to above are known in the prior art. Such stud welding apparatus is used, for example, in the automobile industry to weld studs which carry, for example, fixing parts for brake lines; etc to the automobile body.
There are two fundamentally different electric welding processes used in such apparatus, which differ in the method as to how the arc is produced.
The first welding method is described as `arc ignition`. In this method, the stud welding apparatus, together with the welding stud pressed into the stud holder, is first put in contact with the automobile body and a (in some cases low) current passed through the welding stud so as to create a short circuit. The stud welding apparatus is then moved away from the automobile body by a predetermined distance (often shown on a distance indicator on the stud welding apparatus). This movement away from contact produces an arc, which is used for the welding process. After the ignition of the arc, the welding current is increased as required.
In another method, described as `apex ignition`, the welding stud has a small point facing the automobile body. The welding stud is put in contact with the automobile body under full welding current, so that the point rapidly melts because of the high current flowing within it, and the arc necessary for the welding process is created between the welding stud and the automobile body. In this method it is important that the welding stud is moved relatively far away from the automobile body before ignition.
German Specification No. DE-OS 31 05 987 shows a stud welding apparatus in which the loading means consists of a push rod directly attached to a pneumatically-operated thrust piston. As soon as compressed air is delivered to the cylinder in which the thrust piston is disposed, the thrust piston is moved towards the welding stud, hits it and then pushes it against the resilient restoring force of the stud holder into the welding position.
Similar apparatus is shown in German Specification No. DE-OS 26 04 065. Again, a driving rod is provided which is connected to a pneumatic piston and which pushes a welding stud against the resilient restoring force of the stud holder into the welding position. It differs from the preceding example in that in this case a compression spring is additionally provided which is compressed by the movement of the pneumatic piston towards the stud holder, and which returns the driving rod and the pneumatic piston into the starting piston after the stud has been pressed into its holder.
A different kind of stud welding apparatus is shown in German Specification No. DE-GM 85 11 543. This apparatus is designed to carry out the apex ignition process. In order to provide an adequate distance from the surface to be welded, this apparatus is designed so that the stud holder can move back by a greater distance from the welding face. Unlike the previously-described stud welding apparatus, there is no driving rod present, but there a catch is arranged in the feed channel which swings out or is swung out by force when a stud is inserted into the feed channel and against which the stud strikes when the stud holder is moved towards this catch. The stud is thus stopped by the catch and the force required for the pressing-in of the welding stud is provided by the backmovement of the stud holder.
Known stud welding apparatuses have disadvantages which arise from the kinetics of the loading process. In virtually all the prior art, the pneumatic piston first accelerates the driving rod or else the stud holder until it finally abuts against the welding stud. At this point the piston, together with the driving rod connected thereto, (the stud holder in the DE-GM 85 11 543) has maximum kinetic energy. At the same time, the force which is required in order to open up the stud holder against its resilient restoring forces is, in this position, according to the generally accepted principles of elasticity, at its lowest. The piston, together with the driving rod (the stud holder) is slowed down on contact with the welding stud and pushes the welding stud with decreasing force against the simultaneously increasing resilient forces restoring the stud holder into the welding position. The force exerted for this pushing thus declines during the pushing, whilst the force required for the pushing increases. In DE-OS 26 04 065 this disadvantageous characteristic is even intensified by the spring acting against the piston. On account of this characteristic the stud often does not reach the correct pre-calculated end position in the stud welding apparatus. In particular, when using arc ignition it is necessary that the position of the welding stud in relation to the welding face should be precisely fixed (in the order of magnitude of one tenth of a millimeter) in order to obtain welded joints having reproducible stability characteristics.
Furthermore, known stud welding apparatuses have the disadvantage that they are comparatively long, which is frequently referred to in the prior art. The extensive length of stud welding apparatuses jeopardises the use of these apparatuses for example in narrow spaces and also makes their use more difficult when manually operated.
The object of the present invention is therefore to provide a stud welding apparatus which makes precise insertion of the welding stud into the stud holder possible.
A further aspect of the object of the invention is to provide a stud welding apparatus which itself occupies only little space and which can be used in narrow spaces.